The present invention relates to a liquid crystal display device and more particularly to a driving circuit for driving a liquid crystal display device.
In the case of time multiplex driving of a liquid crystal display device, the amplitude-selective addressing scheme is usually used as described in U.S. Pat. No. 3,976,362 to Kawakami and the polarity of voltage applied to liquid crystal layer is periodically reversed so that the liquid crystal layer has no mean DC level applied to it. For polarity inversion, there are two kinds of methods, one of which is to convert the driving waveforms into alternating waveforms by inverting the polarity within one frame period (the time necessary to scan all scanning lines once), and is hereafter referred to as driving method A, and the other is to convert the driving waveforms into alternating waveforms by inverting the polarity within the period of two frames and is hereafter referred to as driving method B. These methods of time multiplex driving for liquid crystal display elements are discussed in detail, for example, in the Nikkei Electronics, Aug. 18th, 1980, pp 150-174.
The time multiplex driving for liquid crystal display elements is described in the above-mentioned patent and reference, and at present the driving method B is used mainly with the increase of scanning line numbers for time multiplexing in order to avoid the increase of power consumption of a driver LSI.
However, since the lowest driving frequency in the driving method B is the half of the frame frequency, e.g. 70 Hz, there may be a case where liquid crystal display elements are driven at a very low frequency according to a pattern to be displayed. On the other hand, the threshold voltage of the liquid crystal has a characteristic dependent on the frequency of applied voltage, and in case that the threshold voltage of the liquid crystal, a voltage at which ON-state of liquid crystal display elements begins to be visible, falls largely at lower frequencies, strong blurs occur in display according to particular display patterns when the driving method B is employed. For example, if the liquid crystal has a characteristic in which the threshold voltage V.sub.th drops at lower frequencies as is shown in FIG. 1, and the alphabet E is displayed by applying voltage between signal electrodes C.sub.1, C.sub.2, . . . , C.sub.20 and scanning electrodes R.sub.1, R.sub.2, . . . , R.sub.27 selectively as in FIG. 2, darkening of the shaded areas of A.sub.1, A.sub.2 and A.sub.3 occurs, and the degree of darkening is lower than that of the selected element D on B.sub.1 and B.sub.2 areas but higher than that of the non-selected areas E on B.sub.1 and B.sub.2. As a result, dark shades appear near an intended display as shadows. This phenomenon can be explained as follows. The frequency components of the driving voltage V.sub.o applied to the liquid crystal display elements on the areas of A.sub.1, A.sub.2 and A.sub.3 are extremely lower than those of the driving voltage V.sub.o applied to the liquid crystal display elements on the areas of B.sub.1 and B.sub.2. Considering the frequency dependence of the threshold voltage shown in FIG. 1, the voltage V.sub.1 applied to the elements on A.sub.1, A.sub.2 and A.sub.3 areas with respect to their threshold voltages at their frequency is higher than the voltage V.sub.2 applied to the elements on B.sub.1 and B.sub.2 areas with respect to their threshold voltages at their frequency, and as a result, the degree of darkening of the elements on A.sub.1, A.sub.2 and A.sub.3 areas is higher than that of the non-selected elements on B.sub.1 and B.sub.2 areas and the phenomenon of blurs occurs around the display. As an example, the driving waveforms are shown in FIGS. 3(a) to 3(j) which are applied to the display elements a.sub.1, a.sub.2, a.sub.3 and a.sub.4 shown on FIG. 2 by the driving method B. In these figures, by comparing the driving waveforms applied to the display element a.sub.2 with the driving waveforms applied to the remaining display elements a.sub.1, a.sub.3 and a.sub.4, it can be understood that the frequency components of the driving waveforms applied to the display element a.sub.2 is extremely higher than the frequency components of the driving waveforms applied to the display elements a.sub.1, a.sub.3 and a.sub.4, and from the relationships shown in FIG. 1, it can be understood easily that the blurs in display become excessively conspicuous with the increase of frequency range of the driving waveforms. Further, in FIG. 2 the B.sub.1 area appears blanched compared with B.sub.2 area due to the higher frequency components for the B.sub.1 area, and this phenomenon can be explained in the same way as above. Further, in FIG. 3 the symbol .tau..sub.D designates a pulse width of a scanning signal.
As a measure to solve this problem, it may be considered to use the driving method A, but it is known that a different type of blurs in display appears. By driving method A, it can be considered that the blurs are caused by considerable influences of waveform distortions on effective voltage values at resultant higher driving frequencies.